The present invention relates generally to gas turbine engines, and, more specifically, to augmented turbofan engines.
The typical turbofan gas turbine aircraft engine includes in serial flow communication a fan, compressor, combustor, high pressure turbine (HPT), and low pressure turbine (LPT). Inlet air is pressurized through the fan and compressor and mixed with fuel in the combustor for generating hot combustion gases.
The HPT extracts energy from the combustion gases to power the compressor through a corresponding drive shaft extending therebetween. The LPT extracts additional energy from the combustion gases to power the fan through another drive shaft extending therebetween.
In the turbofan engine, a majority of the pressurized fan air bypasses the core engine through a surrounding annular bypass duct and rejoins the core exhaust flow at the aft end of the engine for collectively providing the propulsion thrust for powering an aircraft in flight.
Additional propulsion thrust may be provided in the engine by incorporating an augmentor or afterburner at the aft end of the engine. The typical afterburner includes a flameholder and cooperating fuel spraybars which introduce additional fuel in the exhaust discharged from the turbofan engine. The additional fuel is burned within an afterburner liner for increasing the propulsion thrust of the engine for limited duration when desired.
A variable area exhaust nozzle (VEN) is mounted at the aft end of the afterburner and includes movable exhaust flaps. The flaps define a converging-diverging (CD) nozzle which optimizes performance of the engine during non-augmented, dry operation of the engine at normal thrust level, and during augmented, wet operation of the engine when additional fuel is burned in the afterburner for temporarily increasing the propulsion thrust from the engine.
Flameholders have various designs and are suitably configured to hold or maintain fixed the flame front in the afterburner. The exhaust flow from the turbofan engine itself has relatively high velocity, and the flameholder provides a bluff body to create a relatively low velocity region in which the afterburner flame may be initiated and maintained during operation.
One embodiment of the flameholder that has been successfully used for many years in military aircraft around the world includes an annular flameholder having a row of flameholder or swirl vanes mounted between radially outer and inner shells. Each of the vanes has opposite pressure and suction sidewalls extending axially between opposite leading and trailing edges.
The aft end of each vane includes a generally flat aft panel facing in the aft downstream direction which collectively provide around the circumference of the flameholder a protected, bluff body area effective for holding the downstream flame during augmentor operation. In one embodiment, the aft panel includes a series of radial cooling slots fed with a portion of un-carbureted exhaust flow received inside each of the vanes for providing cooling thereof during operation.
Since the flameholders are disposed at the aft end of the turbofan engine and are bathed in the hot exhaust flow therefrom they have a limited useful life due to that hostile thermal environment. Furthermore, when the afterburner is operated to produce additional combustion gases aft therefrom further heat is generated thereby, and also affects the useful life of the afterburner, including in particular the flameholder itself.
An additional problem has been uncovered during use of this exemplary engine due to the introduction of fuel into the flameholder assembly. This exemplary afterburner includes a row of main fuel spraybars and a fewer number of pilot fuel spraybars dispersed circumferentially therebetween. For example, each vane may be associated with two main spraybars straddling the leading edge thereof, and every other vane may include a pilot spraybar before the leading edge thereof.
The pilot spraybars are used to introduce limited fuel during the initial ignition of the afterburner followed by more fuel injected from the main spraybars. The pilot fuel is injected against the leading edges of the corresponding pilot vanes and spreads laterally along the opposite sidewalls of the vanes prior to ignition thereof.
Experience in operating engines has shown that the relatively cold pilot fuel creates thermal distress in the pilot vanes during operation, and limits the useful life thereof. All the flameholder vanes, including the pilot vanes, operate at relatively high temperature especially during afterburner operation, and the introduction of the pilot fuel introduces corresponding temperature gradients in the pilot vanes which increase thermal stress therein.
Accordingly, the cyclical operation of the afterburner leads to greater thermal distress in the pilot vanes than the other, non-pilot vanes and can eventually induce thermal cracking in the leading edge region of the pilot vanes. These cracks then permit ingestion of pilot fuel inside the pilot vane and undesirable combustion therein which then leads to further thermal distress, spallation, and life-limited damage to the aft panels of the pilot vanes.
It is therefore desired to provide an improved afterburner flameholder for increasing the useful life thereof.